This invention relates to amplification of signals in multi channel optical communications systems, and in particular to the amplification of groups of WDM channels within a WDM optical communications network.
Amplifiers, such as Erbium doped fiber amplifiers, are widely used in optical communications systems, in order to compensate for fiber and splitting losses within the system. The gain response of these amplifiers is wavelength dependent, so that if an amplifier is used for all channels, different channels are amplified with different levels of gain. The wavelength dependency also depends on the gain setting of the amplifier. Gain control schemes typically analyse the average power over the full range of channels, so do not provide gain control on an individual channel basis. Instead, gain flattening filters are used in an attempt to bring the signal intensity to the same level for all channels.
It has also been proposed to provide individual amplifiers for groups of channels, known as sub-bands. This enables dispersion compensation and power management to be designed for each sub-band. These sub-bands are typically used at multiplexing and demultiplexing sites, where the sub-bands exist as an intermediate level of multiplexing between the fiber and the individual channels. Thus, sub-bands may be employed at transmit and receive nodes, as well as at add-drop sites.
The amplification of sub-bands enables the gain flatness to be controlled more accurately across that group of channels. Different gain settings can then also be applied to different amplifiers, to achieve gain flatness across the different groups of channels. In a practical implementation, a variable optical attenuator can be associated with each amplifier. However, the losses of the VOA result in the amplifiers being pump power inefficient, and the achievable noise figure of the amplifier is degraded. Each amplifier is associated with a dedicated gain flattening filter, and a problem with this approach is the expense of providing different fixed filters for each sub-band.
A further alternative is to use identical filters for the gain flattening of each sub-band. To enable this, the filter needs to have a loss characteristic which maps the amplifier characteristics across the full channel spectrum. This inevitably results in the filter imposing more loss on some bands of channels than would be the case with a dedicated filter. The use of a single filter design thus reduces the implementation cost, but at the expense of reduced pump power efficiency.
There is therefore a need for an efficient and low cost gain flattened amplifier design.
According to the invention, there is provided an optical amplifier for amplifying the channel signals of a WDM signal, comprising an amplifier arrangement for amplifying a band of the channels, the amplifier arrangement comprising an amplifier and a filter, the filter comprising a controllable optical filter having an attenuation characteristic which comprises a region of positive gradient and a region of negative gradient with respect to input frequency, wherein the magnitude and alignment along the frequency axis of the attenuation characteristic is variable, such that the filter can be tuned to provide gain flattening for the amplified band of channels.
The filter design used in the invention can be tuned to the requirements of a particular sub-band, whilst identical filters can be used for each sub-band. This invention enables identical filters to be used, and gives good pump power efficiency, as the attenuation of each filter can be controlled to the desired level, thereby avoiding the need for large increases in pump power.
The attenuation characteristic is preferably periodic with input frequency, and the magnitude and phase of the attenuation characteristic is then variable. The amplifier may further comprise a first multiplexer arrangement between the fiber carrying the WDM signal and a plurality of signal carriers each carrying a respective band of channels, and a second multiplexer arrangement between the signal carriers for the bands and signal carriers for the individual channels, wherein each band is provided with an amplifier and an identical filter. Thus, identical filters can be used for different bands.
Each filter may comprise a sinusoidal filter, and may include an input wave guide, an output waveguide, a polarisation separation means, a variable polarisation rotation means, the variation being dependant on a first control input signal, and a birefringent element having variable birefringence, the variation being dependant on a second control input signal, wherein an optical input signal emitted form the input waveguide passes through the polarisation separation means, the variable polarisation rotation means and the birefringent element to the output waveguide so as to produce at the output waveguide an optical output signal having a frequency dependant output characteristic determined by the two control input signals.
The magnitude of the attenuation can be determined by the first control input signal and the phase of the attenuation can be determined by the second control signal.
The amplifier arrangement may comprise an Erbium doped fiber amplifier, or a plurality of such amplifiers.
According to a second aspect of the invention, there is provided a method of providing gain-flattened amplification of at least one band of channels of an optical WDM signal, comprising:
forming bands of channels;
amplifying at least one band of channels and applying filtering using a controllable optical filter having an attenuation characteristic which comprises a region of positive gradient and a region of negative gradient with respect to input frequency, wherein the step of applying filtering comprises setting the magnitude and alignment along the frequency axis of the attenuation characteristic, such that the filter is tuned to provide gain flattening for the amplified band of channels.
The step of forming bands of channels may comprise demultiplexing the channels from a fiber carrying the WDM signal (at a receiving node of a network) or it may comprise multiplexing the channels from individual transmitters for each channel (at a transmitting node of a network).
The invention may be used in a WDM optical communications system.